Lecture 2: Transport and Osmosis
Reading: OpenStax A&P Text Chapter 3.1
Silverthorn: Chapter 3, 5
The cell: A summary
We begin our survey of human physiology by looking at the most significant parts of the cell. Our treatment of the cell assumes a
solid understanding of cell biology gained in a general biology course, like Bio 1. You are welcome (and encouraged) to review Bio
1 lectures on the cell, if you want to brush up on your basics. In this lecture, we will focus on the cell membrane, which mediates
critical interactions between the ECF and ICF.
1. A CELL: the fundamental unit requiring a homeostatic environment (Review Ch 3)
2. There are 100 trillion cells in human body (Guyton, Med Phys 11e pg3) and 25 trillion are RBC. (Perspective: it would take you
3.2 million yrs to count to 100 trillion if you count 1 #/sec!)
3. 200 different cell types in human body (based on selective gene activation)
4. The cell membrane is a semipermeable separation between ICF and ECF.
5. Fluid compartments are separated by cell membranes, so cell membranes mediate interactions between fluid compartments.
6. The cell membrane is composed of a phospholipid bilayer that is studded with many different types of molecules, including:
A. Glycoproteins and glycolipids (involved in immunity and cell recognition)
B. Cholesterol chills within the lipid layer, providing structure and impermeability to water soluble molecules. The more
cholesterol found in the cell membrane, the more “water tight” the cell becomes.
C. Many embedded proteins (The more proteins there are, the more metabolically active the cell)
Proteins found in the cell membrane
1. Structural proteins (connect cells together, keep shape, attach cell to extracellular matrix…)
A. Gap junctions (connexins): Simple tunnel between 2 cells, like a rivet; holds them together AND allows stuff through.
B. Tight junctions (claudins, occludins): Connects two cells, but does not allow passage between cytoplasm; can be leaky…
2. Enzymes catalyze chemical reactions and can be attached to outside surface or the inside surface
3. Receptors are cellular protein that binds to a ligand (a molecule that binds to a protein…hormone, NT, ion…)
A. Can be located on cell membrane…also on DNA…also can float!
B. Molecules (like hormones) bind to receptors, triggering a RESPONSE…
4. Transporters allow molecules to pass through the cell membrane.
A. Channel proteins enable facilitated diffusion
B. Carrier proteins can enable facilitated diffusion or active transport
Molecular structure determines function
1. Proteins are molecular machines and the way they are SHAPED determines what they can DO.
i. Primary structure: String of amino acids
ii. Secondary structure: The string is folded in some way (beta pleated sheets or alpha helices)
iii. Tertiary structure: The folded shape FOLDS on itself
iv. Quaternary structure: More than one protein with tertiary structure COMBINES
A. Exposure to acids or bases (or heat) can affect the ability of each amino acid to form chemical bonds with others.
B. Even exposure to different concentrations of ions can affect the shape of a protein.
C. Binding with other substances can also affect the shape of a protein...(example: transporters!)
2. Enzymes are proteins that catalyze chemical reactions
3. Receptors are proteins that bind to signal molecules (like hormones) and then initiate some sort of cellular response.
4. Transporters are proteins that help substances move in and out of a cell.
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Diffusion
Diffusion is one way to transport “stuff” through the cell membrane. SIMPLE DIFFUSION is the random movement of molecules
from areas of high concentration to areas of lower concentration.
1. Requires no input of energy (passive)
2. Happens when particles move DOWN their concentration gradients.
3. Relies simply on the kinetic energy of the particles in question.
4. Stops when equilibrium is attained.
5. Direct DIFFUSION through cell membrane is restricted to molecules that are:
A. Lipophyllic (hydrophobic)
B. Small… (Ex: Oxygen, CO2, Nitrogen and alcohol)
C. (Note: water CAN diffuse through the membrane, if there is not much cholesterol in the membrane, even though it is
lipophobic, BUT this is pretty slow…)
↓ conc water
↑ conc water
↑ conc solute
↓ conc solute
Facilitated diffusion
FACILITATED DIFFUSION requires a protein helper to allow a molecule to make it through the cell membrane. There are a couple
of types of proteins that enable movement of molecules in this way.
1. CHANNELS are water-filled tunnels that directly link ECF and ICF.
A. Usually the channel is SPECIFIC for a molecule
B. Diffusion can be very fast
i. Example: Aquaporins (Water channels)
ii. In 1 second, the amount of water that diffuses (in each direction) through the cell membrane of a RBC is about 100x the
volume of the cell!
C. Example: Ion channels
i. Very specific to the substance that can go through…
ii. Can be gated (closed or opened based on stimulus, including voltage, a chemical signal/enzyme, or even pressure/temp
stimuli)
iii. Gated channels are CONDITIONAL…
iv. Example: Gated Na+ channel, which is 0.3x0.5 nanometers in diameter and has an inner surface that is VERY
NEGATIVELY CHARGED! This negative charge pulls Na+ away from their hydrating water molecules and into the
channel. This allows the ions to diffuse down conc gradient.
v. Compare to the gated K+ channel which is 0.3x0.3 nanometers in diameter and has an inner surface that is NOT
NEGATIVELY CHARGED! This lack of negative charge keeps Na+ out b/c the hydrated Na+ is too big to go through
(and the hydrated K+ is much smaller than hydrated Na+…)
I. K+ diffuses down conc gradient.
2. TRANSPORTERS can engage in facilitated diffusion if they move substances DOWN a concentration gradient…
A. Protein carrier provides a way through the cell membrane
B. Does NOT require direct energy
C. Is open to one side or other, never form a direct link between ECF and ICF
D. Is SLOWER than a channel, b/c it must open and close
E. Example: GLUT
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Active Transport
Active transport happens when energy is required to pump molecules AGAINST a concentration gradient. There are two ways
transporters get energy to make this happen.
1. Primary active transport moves molecules against conc gradient. This requires a direct energy source, usually in the form of
ATP.
A. An example: Na+/K+ pump! (a membrane protein)
2. Secondary Active transport also moves molecules against conc gradient, but does not require a direct energy source, though
DOES require another molecule’s concentration gradient.
A. The energy comes from another molecule going DOWN its concentration gradient
B. Example: (SGLT)
3. Phagocytosis/ endocytosis and exocytosis does require energy…
A. Does NOT necessarily require a transport protein (though there is a receptor that triggers the chemical reaction taht results
in the process).
Osmosis
1. Osmosis is the movement of water molecules across a semi-permeable membrane... (http://highered.mcgraw-hill.com/
sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html)
2. Movement of water can create OSMOTIC PRESSURE!
A. Conclusion: Osmolarity describes the solution, but not how WATER will affect the cell…what the water does depends on:
i. Osmolarity of both “sides”
ii. Qualities of the MEMBRANE separating the sides.
iii. What particles can pass through, and what particles cannot?!
a. # Penetrating solutes on each side (can pass through the membrane)
b. # Non-penetrating solutes on each side(cannot pass through the membrane)
B. Tonicity describes how WATER will affect a cell…
i. In a hypotonic solution, the water rushes into the cell, and it swells.
ii. In a hypertonic solution, the water rushes out of the cell, and it shrinks (crenates)
iii. In an isotonic solution, the water does not move…the cell stays the same size.
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Lab 2: Rate of Osmosis
Materials:
Part 1: The Rate of Osmosis
6 dialysis bags
Thread
6 (400 mL) beakers
Tap water
400 mL 0.28M NaCl
10 mL of each soln
Digital scale
Paper towels
5mL pipettes
Green pipettor
Wax pencil
Procedure:
1. Obtain 6 (400 mL) beakers and use a wax pencil to label them 1-6.
2. Fill beakers 1-5 with about 400 mL of tap water, and fill beaker 6 with about 400 mL of 0.28 M NaCl solution.
3. Carefully tie one end of each dialysis bag with thread (as described in class, with 2 hot dog folds followed by a hamburger fold).
4. Fill each dialysis bag with 10 mL of the solutions indicated in the following table. Carefully remove the air from the bag, then tie
the other end, being sure to leave room for the bag to expand.
5. Dry each bag and determine its weight. Record this number in the cell entitled “actual mass” under “Time = 0.”
6. Carefully place the dialysis bag into the correctly numbered beaker. Record your start time in the table.
7. Remove the bags at 15 minute intervals and dry and weigh them. Find the difference in mass between the current time and the
previous time to determine the “change” in mass. If there is a loss in weight, indicate that with a negative sign.
Dialysis bag Solution
in bag
(10 mL)
Solution
in beaker
(400 mL)
1
Water
Water
Hypothesis: Describe the
tonicity of the solution
compared to the bag.
Mass of bag (in grams)
Time = 0
Time =
15 min
Time =
30 min
Time =
45 min
Time =
60 min
Actual mass
Start time
Change in mass from
the previous time
2
Start time
0.28 M
glucose
Water
Actual mass
Change
Is this a
penetrating
solute?
3
Start time
0.28 M
Water
glycerine
Actual mass
Change
Is this a
penetrating
solute?
4
Start time
0.28 M
urea
Water
Actual mass
Change
Is this a
penetrating
solute?
5
Start time
0.28 M
NaCl
Water
Actual mass
Change
Is this a
penetrating
solute?
6
Start time
Water
0.28 M
NaCl
Actual mass
Change
Is this a
penetrating
solute?
Report all data on the class Google Doc in Canvas.
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Lab Questions
1. Describe the concept of a “semi-permeable” membrane in relation to your lab experiments.
2. Each dialysis bag/beaker setup is one mini experiment. In each experiment, what is the independent variable? The dependent
variable?
3. Which experiment represents your control?
4. Why does it matter whether or not the solute in each solution is “penetrating?”
5. Graph your data. Remember that the dependent variable should be graphed on the y axis. Make sure it has 0 in the middle of the
axis, allowing you to graph positive changes in weight above the 0 line and negative changes below the 0 line.
6. What results would you expect if you’d filled one dialysis bag with a 0.56 M solution of NaCl? Why would you expect this?
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Osmosis Worksheet: BONUS
The cell membrane is semipermeable. Penetrating solutes include urea and glycerine. They are considered penetrating because they
can easily pass through the cell membrane. Non-penetrating solutes include NaCl and glucose. The concentration of urea, glycerine,
glucose and NaCl inside the cell is negligible.
Most cells in the human body have a concentration of 280 mOsm in the intracellular fluid (ICF). You can assume this is always true,
unless you are TOLD otherwise. Most solutes found in the ICF are non-penetrating. If you think about this, it makes sense. If the
particles in the ICF were penetrating, then they would just diffuse OUT of the cell and into the extracellular fluid, until equilibrium is
reached. Therefore, if they are found INSIDE the cell, they must be either trapped there, or also present in the solution surrounding the
cell.
In this exercise, NaCl is the only substance that dissociates into 2 particles in water. Remember, this means that a 200 mM solution of
NaCl is actually a 400 mOsm solution!
Directions: For each scenario below, you will imagine you have a beaker of solution, to which you will add a single normal cell.
Then you will fill in the following chart, focusing on two things:
A. What is the osmolarity of the solution compared to cell, BEFORE dropping in the cell into the solution?
B. Imagine you add the cell to the solution, then allow osmotic equilibrium to be established between the cell’s ICF and
the solution in the beaker. Describe the TONICITY of the SOLUTION based on how it affects the cell (after osmotic
equilibrium is reestablished).
Molarity of
Osmolarity
Is the solute
Osmolarity of The solution is ___
solution in
of solution in
in the solution the cell (before osmotic (hyper-,
beaker (before the beaker (before the penetrating? the cell is dropped is-, or hyp-)to the
into the solution) cell (before the cell
cell is dropped into cell is dropped into
the solution)
1
the solution)
is dropped into the
solution)
The solution is
___tonic (hyper-,
iso-, or hypo-)
to the cell (after
osmotic equilibrium is
reestablished)
280 mM
glucose
2
280 mOsm
3
glucose
280 mOsm
urea
4
200 mM
NaCl
5
200 mOsm
6
NaCl
280 mOsm
glycerine
7
10 mM
urea
8
10 mOsm
9
glycerine
10mOsm
glucose
10
280 mM
NaCl
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External Brain 2: Transport and Osmosis
Complete all the following tasks on a separate sheet of paper and include it in your External Brain binder. Remember the rules for the
External Brain. All work must be your OWN, although you may use UNLABELED images if you want. Make sure you cite all your
sources!
Study Guide Questions
1. Describe the structure and function of the cell membrane. Your answer should be specific and detailed.
2. Describe several mechanisms by which molecules move through the cell membrane. Clearly connect the general type of
molecules with the mechanism of transport.
3. Given several cells with different numbers of proteins embedded in the membrane, be able to determine the relative rates of
metabolic activity for each cell. (In other words, label the cell as more or less metabolically active...)
4. Be able to describe different ways cells are connected.
5. Compare and contrast the following transporters:
a. SLGT
b. GLUT
c. Na+/K+ Pump
d. Ion channels
6. List the functions of proteins and other molecules embedded in the cell membrane.
7. Compare and contrast diffusion, active transport, and facilitated diffusion.
8. Compare and contrast primary and secondary active transport.
9. Compare and contrast endocytosis and exocytosis. What kind of transport is this?
10.What is epithelial transport?
11. Draw a single epithelial cell. Label the side that faces the external environment and the side that attaches to the body’s
connective tissue.
12.Draw a picture of a substance being transported across an epithelial cell. Include the transporters that would be necessary to
make this happen. Defend your answer. (Hint: Usually one side of the cell actively transports materials into/out of the cell
and the other side makes use of passive (or facilitated) transport.
13.Can a solution be HYPERTONIC to a cell before you drop the cell into the solution?
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